This invention relates to methods and compositions to decrease or remove iron sulfide deposits in or on a conduit of a gas stream.
Hydrogen sulfide (H2S) is a pernicious, naturally occurring contaminant of fluids that is encountered, for example, during the manipulation of oil or gas. The corrosive nature of H2S typically leads to the accumulation of particulate iron sulfide, which can become easily entrained in hydrocarbons as well as in glycol, salts, and other contaminants, forming intractable deposits on the surfaces of conduits such as pipelines. Such deposits present a significant problem to the oil and gas industries because the pipelines must be cleaned physically. Additionally, the iron sulfide deposits hinder accurate determinations of pipeline structural integrity, which can be assessed by instrumentation known as SMART PIGS.
A limited repertoire of techniques has been available for reducing or removing iron sulfide deposits from pipelines. U.S. Pat. No. 5,820,766 to Gevertz et al., for example, describes the use of inorganic bromates or iodates to oxidize fluid-entrained sulfides to solid elemental sulfur, which must be mechanically collected and removed from a pipeline. A byproduct of this mechanical cleaning is a sludge that can be flammable and must be disposed of in a landfill. U.S. Pat. No. 4,370,236 to Ferguson discloses a method in which iron sulfide is removed from a gas stream by washing it with a mixture of a hydrocarbon and water. The resultant aqueous phase contains soluble and particulate iron sulfide which must be removed by further physical and chemical processing steps. For example, see U.S. Pat. No. 6,153,100.
An alternate approach as informed by general chemical principles is to solubilize iron sulfide in water. Iron (II) and iron (III) ions generally form relatively water-insoluble compounds at neutral pHs. Such iron compounds begin to precipitate from aqueous solution at pH values of 5 or greater. For example, iron (II) precipitates from neutral solutions at pH 7 and oxidizes to iron (III) hydroxide in the presence of oxygen. Thus, the usual method to render a water insoluble iron (II) or iron (III) compound soluble in water is to treat the solid in an aqueous mixture with a strong mineral acid which lowers the pH, thereby dissolving the iron compound. In the case of iron (II) sulfide, however, this method results in the evolution of hydrogen sulfide, and if in sufficient amount (greater than 437 cm3/L at 0° C.) to its release as a toxic gas from the solution. An additional disadvantage of using of strong mineral acids to clean pipelines is that most pipes are made of steel or iron, which are susceptible to attack by strong acids, thereby producing corrosion, deterioration, and pitting. Furthermore, such attack also produces hydrogen gas, which is flammable and explosive in air.
Yet another approach to the removal of iron sulfide is disclosed in PCT publication WO 02/08127, which describes the use of aqueous compositions of tris(hydroxymethyl)phosphine or a corresponding phosphonium salt (collectively, “THP”) below or well below neutral pH. The '127 PCT publication discloses, however, that the use of THP, at the pH required to rapidly complex iron sulfide, is fraught with practical barriers, including the formation of an insoluble polymer, when THP is formulated with ammonia as a co-reagent, and the oxidation of THP to the non-complexing tris(hydroxymethyl)phosphine oxide. In light of these problems, the '127 PCT publication discloses that iron sulfide can be chelated by amino carboxylic acids or amino phosphonic acids in formulations with THP. According to the publications, the use of THP in the absence of ammonium ion or ammonia provides a small synergistic effect on iron sulfide dissolution. Because the acid co-reagents are expensive, however, their use is undesirable when large quantities are necessary to remove iron sulfide deposits.
Accordingly, there is a continued need in the art for an improved method of removing iron sulfide deposits that employs safe, readily available and inexpensive materials, which requires minimum mechanical intervention, and that avoids chemical pitfalls, such as polymeric precipitates, of prior art methods.